The use of a separate mask or template or stencil to control the pattern of a material deposition on a portion of a structure is well known. Openings are provided in the sheet of mask material in a pattern which corresponds to the desired pattern to be imparted onto the structure. In most cases, openings are provided on selected portions of the mask through which material depositions are to be made. Such a mask is placed against the structure surface and deposition on the selected portions through the mask openings are made. The materials that are deposited on the structure are approximately of the desired geometrical arrangement. The structure itself may have openings through which the deposited material may pass through or the material may be deposited within those openings. However, the final geometrical pattern of deposited material on the structure following this deposition may differ from the desired geometrical pattern to some extent. This may be due to some of the material getting deposited underneath the mask and along the edges of the openings therein during deposition. This may occur because the sheet of mask material may not be uniformly in contact with the structure surface, thereby leaving gaps between the mask and the structure surface. This may also occur if excessive amounts of deposition material is poured onto the mask and the excessive material overflows onto the structure. The final geometrical pattern may also be different when the mask clings onto the structure, e.g., because of suction that may have been created between the structure and the mask, or, because the deposition material acts like glue, and upon separation deformities and discontinuities occur, as will be discussed later. Even if the method for depositing material on the structure is changed, the new method cannot be expected to alter greatly the result that some deposited material does not create the desired geometrical pattern.
Typical structures upon which material depositions are desired to be made are integrated circuit wafers, greensheets, and substrates which are used to mount thereupon various kinds of electronic devices. The surfaces of these structures against which a mask is to be placed are usually very flat. This, in turn, requires that the mask be very flat against this surface if there is to be uniform contact and gaps are not to occur and that the desired geometrical pattern is obtained.
The gaps between the structure surface and the mask may occur when the structure surface and the mask are forced against one another. If the acting forces are not uniform on the mask, then there is a "bowing" effect on the mask, even though the structure and the mask are symmetrically aligned with one another and even though the mask and the structure surface are both flat and parallel. These gaps may also occur because of the bending of the mask due to the resultant forces acting on it or due to the structure surface pressing against it or due to the mask holder constraining the edge of the mask. Failure of symmetrical and/or parallel alignment or deviations in flatness in the mask sheet itself, all difficult to avoid, are further sources of aggravations for such gaps. When such gaps occur for flat sheet masks which are held by a holder, the deposited material or paste may creep in through the mask openings and into the underside of the mask.
Additionally, when the desired material has been deposited on the structure and the structure is separated from the mask, the material adhering to the underside of the mask and the structure, forces the mask to stick to the structure and move along with it as the structure is being separated or pulled away from the mask. The edges of the mask separate earlier from the structure and the center of the mask area is usually the last mask surface to separate. Prior to complete separation, the center area of the mask creates a bowing configuration, because it is sticking onto the surface of the structure while the rest of the mask surface has separated from the structure.
The material to be deposited usually has a very high degree of viscosity. The deposited material on the surface of the structure forms a continuous contact with the deposited material on the mask through the mask openings and results in an adhesive type of a contact. Due to the high viscosity of the deposited material and the adhesiveness of the deposited material, the mask, which is normally a metal mask, snaps away from the structure upon separation thereby pulling some of the deposited material away from the structure. This "snapping" process creates discontinuities or deformities. Sometimes when the mask separates from the structure, it leaves behind spikes in the pattern, thereby creating a defective structure, as those spikes can result in a short. This happens because the spike usually falls to the side and creates a short between the adjacent lines or geometrical patterns. It has also been found that the spiking level in the center of the structure is significantly higher than that of the spiking level in the perimeter of the structure. The spikes or deformities can also have an adverse effect on any subsequent deposition or on any additional layers that may be placed over it. It is, therefore, desirable to have the mask and the structure surface placed in contact with one another in such a way as to minimize the formation of such gaps and/or spikes and to control the separation of the structure from the mask.
The mask, typically a metal mask, made from, e.g., copper or aluminum or molybdenum sheets or alloys thereof, is attached to a holder. The mask presently in use is made of very thin metal sheets. The mask and the holder are usually held in a stationary position while the structure is allowed to move. Forces that are applied from the structure to the mask result in the mask being flexed into a protrudent shape with resulting substantially convex major surface directed away from the surface of the structure or wafer against which the mask is to form a seal. The protrudent shape of the mask is modified to some extent by the force occurring between the structure and the mask when they are brought together.
In depositing inlaying composition or material through the mask openings, there is a tendency for the paste or similar composition to cling to the mask when the mask is lifted, and the composition sometimes builds up in the mask openings resulting in the production of defective structures. Therefore, the mask has to be kept clean so that this buildup does not adversely contribute to the deformities and discontinuities that are produced during the separation of the mask from the structure.
One way to separate the structure from the mask is to strike a sharp blow, preferably near the center. This will induce a shock wave in the mask, the wave will travel along the mask, and the structure will part from the mask. If this action is imparted to the mask upon completion of each masking operation, there will be no tendency for the structure to adhere to the mask; and thus defective structures may not be produced. This method of separating the mask from the structure may result in damage to the mask and may also result in forcing some of the deposition material to fall onto the structure or be forced into the opening within the structure, thereby making the structure defective again.
In the past one way to solve the problem was to control the machine velocity for the entire mask/structure separation process. This was done either by providing uniform machine velocity or by decreasing the separation velocity. Both of the methods do result in decreasing the number of spikes but they do not eliminate the spiking problem. Decreasing the separation velocity also had a detrimental effect on the thruput capacity of the screening equipment.
Another way of correcting this type of a spiking problem would be to evaluate paste changes. From the studies conducted, it was determined that a paste formulation change is not the answer to the spiking problem.
The present invention reduces the screening process cycle time because the mask movement prior to and after the separation from the structure is controlled. The mask shock absorbing system will eliminate the mask "snapping" during the mask/structure separation. This controlling of the mask during the mask/structure separation will also eliminate spikes and paste ponds due to mask "snapping". This, of course, significantly increases the percentage of structures or greensheets that are not defective because of the "snapping" action of the mask.